A well casing is normally disposed within a vertically oriented well bore and sealed therein by pumping cement into the annular space between the outer surface of the casing and internal wall of the bore. The well casing may be made of metal or synthetic plastic material and can extend for several miles below the ground surface.
In order to maximize the extraction of hydrocarbon values from a well, it is often necessary to provide a series of lateral perforations through the well casing, adjacent casing cement and bore formation material. However, the procedure of accurately positioning a perforating tool in the desired vicinity of the casing to be perforated, and reliably effecting the perforation without damaging the well structure, is often a very critical and difficult task to accomplish, particularly if the perforations are to be placed thousands of feet below ground level.
The prior art has sought to accomplish the accurate and reliable perforating of well casings through the use of two general types of perforating tools. First, it is known to use ignitable charges, such as a gun that fires a bullet propelled by an explosive propellant. The gun is lowered into the vicinity of the casing that is desired to be perforated and, upon actuation of the gun from an above-ground control signal, the bullet is fired and caused to penetrate the well casing and adjacent cement and bore formation. This type of perforating device may also utilize chemical charges, such as thermite or the like, disposed adjacent the inner wall of the well casing so that the desired perforation can be formed by igniting the charge which burns through the casing wall. Devices of this type utilizing an explosive or ignitable composition are basically unreliable in actual operation because of variations in charge compositions and the unpredictable manner in which bullets penetrate the casing and associated formation material. The use of bullets is particularly critical since the explosive impact sometimes causes unexpected damage to the well assembly that is always extremely expensive to repair.
The second type of perforating tool taught by the prior art comprise motor-driven mechanical drills which are normally housed within enclosures that are supported by a cable and lowered into the well casing to the desired location. A basic design problem inherent in tools of this type is the manner in which the dual functions of rotating the drill bit, and advancing and retracting the drill bit with respect to the interior wall of the casing, are effected. Typically, the tool is lowered and locked into position within the well casing and a second cable supporting the motor and associated drill bit is lowered and raised to move the bit towards and away from the casing wall. The drill bit is normally supported at the end of a flexible drill shaft cable that extends from the drive shaft of the motor and bends laterally in a direction towards the interior wall surface of the casing. This basic structure is also found in similar tools utilized for drilling and removing samples from well bore formations.